Current perpendicular to plane, giant magneto-resistance (CPP, GMR) read heads are considered as promising candidates for 180 Gb/in2 and higher magnetic recording densities. This increase in recording density requires the reduction of the read head dimension. For example, for 180 Gb/in2, dimensions around 0.1×0.1 microns are required. A CPP read head can be considered functional only if a significant output voltage, Vout can be achieved when the head senses the magnetic field of a recorded medium. If DR is defined as the resistance change under the magnetic field for the head sensor and I is the current that is sent through the sensor, thenVout=DR×I  (Eq. 1)
Almost all attempts by the prior art to increase Vout have focused on ways to increase DR since it has been assumed that I was already at its maximum value, any further increases being expected to lead to unacceptable increases in the operating temperature of the device. The present invention is directed to ways to increase/without raising the operating temperature of the device above acceptable levels.
To increase application current I is as challenging as to increase DR, especially when the device dimension is getting smaller. This is due to the fact that current density J is inversely proportional to the CPP device dimension and Joule heat is proportional to the square of the current density. Device damage from Joule heat will limit the increase of the application current.
The principle governing the operation of most current magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
Referring now to FIG. 1, we show there the main features of a CPP GMR read head device. These are an antiferromagnetic (pinning) layer 12, a pinned layer 14, a non-magnetic spacer layer 15, a free layer 16 and a capping layer 17. Additionally there may be a seed layer (not shown) directly below layer 12.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance at this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8–20%.
Previously, GMR devices were designed so as to measure the resistance of the free layer for current flowing parallel to the film's plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible while the resistance of the leads into and out of the device need not be particularly low. By contrast, in a CPP device, the resistivity of both the leads and the other GMR stack layers dominate and should be as low as possible.
A related device that is particularly well suited to the CPP design is the magnetic tunneling junction (MTJ) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. The principle governing the operation of the MTJ is the change of resistivity of the tunnel junction between two ferromagnetic layers. When the magnetization of the two ferromagnetic layers is in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability. The change of resistance is typically about 40%.
Returning now to FIG. 1, it can be seen that current enters the device through lead 11a and exits through lead 11b (or vice versa if convention demands). It is important to note that, in prior art devices, 11a and 11b are invariably formed from the same material, most typically copper or gold, selected for their high electrical conductivity. There is no reason to use different materials since this would only ad to the cost.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. Nos. 6,452,740 and 6,105,381, Ghoshal describes devices connected to micro-coolers. Sin et al (U.S. Pat. No. 6,353,318) is an example of the many patents that disclose top and bottom leads composed of the same materials. U.S. Pat. No. 5,627,704 (Lederman et al) and U.S. Pat. No. 5,668,688 (Dykes et al) show a CPP mode read head.